minstrel/internal/logging/recent.go

package logging

import (
	"context"
	"fmt"
	"log/slog"
	"strings"
	"sync"
	"time"
)

// The server's recent errors, kept in memory for the admin Overview (M504):
// what has gone wrong since the process started, without reading the
// container's logs. Only ERROR and above, and only the newest few; the full
// record is still the log stream.

// RecentErrorsKept is how many recent errors are kept.
const RecentErrorsKept = 50

// LoggedError is one error the server logged.
type LoggedError struct {
	At      time.Time
	Message string
	// Attrs is the record's attributes as key=value, the way the text log
	// writes them.
	Attrs string
}

// ErrorRing keeps the newest errors and counts all of them.
type ErrorRing struct {
	mu    sync.Mutex
	buf   []LoggedError
	next  int
	total int64
}

// Errors is the process's ring, filled by every logger New returns.
var Errors = NewErrorRing(RecentErrorsKept)

// NewErrorRing returns a ring that keeps the newest n errors.
func NewErrorRing(n int) *ErrorRing {
	return &ErrorRing{buf: make([]LoggedError, 0, n)}
}

func (r *ErrorRing) add(e LoggedError) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.total++
	if len(r.buf) < cap(r.buf) {
		r.buf = append(r.buf, e)
		return
	}
	r.buf[r.next] = e
	r.next = (r.next + 1) % len(r.buf)
}

// Snapshot returns the kept errors, newest first, and how many have been
// logged in all since the process started.
func (r *ErrorRing) Snapshot() ([]LoggedError, int64) {
	r.mu.Lock()
	defer r.mu.Unlock()
	out := make([]LoggedError, 0, len(r.buf))
	for i := len(r.buf) - 1; i >= 0; i-- {
		out = append(out, r.buf[(r.next+i)%len(r.buf)])
	}
	return out, r.total
}

// ringHandler passes every record on, and copies the errors into a ring.
type ringHandler struct {
	inner slog.Handler
	ring  *ErrorRing
	// attrs are the logger's own attributes (component=… and the like),
	// already rendered, group-qualified.
	attrs  []string
	groups []string
}

func newRingHandler(inner slog.Handler, ring *ErrorRing) *ringHandler {
	return &ringHandler{inner: inner, ring: ring}
}

func (h *ringHandler) Enabled(ctx context.Context, l slog.Level) bool {
	return h.inner.Enabled(ctx, l)
}

func (h *ringHandler) Handle(ctx context.Context, rec slog.Record) error {
	if rec.Level >= slog.LevelError {
		parts := append([]string(nil), h.attrs...)
		rec.Attrs(func(a slog.Attr) bool {
			parts = append(parts, h.render(a))
			return true
		})
		h.ring.add(LoggedError{At: rec.Time, Message: rec.Message, Attrs: strings.Join(parts, " ")})
	}
	return h.inner.Handle(ctx, rec)
}

func (h *ringHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	next := *h
	next.inner = h.inner.WithAttrs(attrs)
	next.attrs = append([]string(nil), h.attrs...)
	for _, a := range attrs {
		next.attrs = append(next.attrs, h.render(a))
	}
	return &next
}

func (h *ringHandler) WithGroup(name string) slog.Handler {
	next := *h
	next.inner = h.inner.WithGroup(name)
	next.groups = append(append([]string(nil), h.groups...), name)
	return &next
}

func (h *ringHandler) render(a slog.Attr) string {
	key := a.Key
	if len(h.groups) > 0 {
		key = strings.Join(h.groups, ".") + "." + key
	}
	return fmt.Sprintf("%s=%v", key, a.Value.Resolve())
}